-
von Rad, U., Haq, B. U., et al., 1992Proceedings of the Ocean
Drilling Program, Scientific Results, Vol. 122
18. FERROMANGANESE DEPOSITS FROM THE WOMBAT PLATEAU,
NORTHWESTAUSTRALIA1
Eric H. De Carlo2 and Neville F. Exon3
ABSTRACT
Ferromanganese crusts, nodules, and ferromanganese-rich
sediments were recovered on the Wombat Plateau,northwest Australian
continental margin, by dredging during Bureau of Mineral Resources
cruise 56 of Rig Seismicand by drilling during ODP Leg 122 of JOWES
Resolution. We report here the chemistry and mineralogy of
theferromanganese crusts, nodules, and associated
ferromanganese-rich sediments. The ferromanganese deposits fromthe
ODP sites are up to 40 cm thick and probably formed in Late
Cretaceous to Eocene times. Those from outcropsusually formed in
several phases, and their age is unconstrained except that the
substrates are Mesozoic. Thesamples were recovered from present-day
water depths of 2000-4600 m, on the Wombat Plateau adjacent to
theArgo Abyssal Plain.
Both the nodules and crusts are primarily vernadite (δ-MnO2) and
are chemically and mineralogically similar, andnot dissimilar from
ferromanganese deposits found elsewhere on Australian and other
marginal plateaus. They aremarkedly different from most deep-sea
deposits. The only crystalline iron phase identified within the
ferromanga-nese deposits is goethite. Concentrations of metals of
potential economic interest are generally low compared tothose from
vernadite-rich seamount crusts and nodules and from abyssal nodules
from areas of high resourcepotential in the Pacific Ocean. Maximum
metal values reach 0.55% Co, 0.58% Ni, and 0.20% Cu in
depositscontaining 4.8% to 30.9% Fe and 4.4% to 21.1% Mn.
INTRODUCTION
Marine ferromanganese deposits are nearly ubiquitous inthe
world's oceans and occur primarily as loose nodules or
asencrustations on hard substrate outcrops (Glasby, 1977).Although
nodules are more common in abyssal settings andcrusts are typically
found on exposed surfaces of seamounts,both types of deposits are
also known to coexist. Hydrother-mal Fe-Mn oxides found near sites
of active venting such asmid-ocean ridges and fore- and back-arc
settings represent anadditional form of Fe-Mn deposit in the marine
environment.
Marine Fe-Mn deposits have been a subject of greatinterest for
several decades because they may represent apotential source of
economically valuable metals such as Co,Cu, and Ni. The literature
is replete with papers discussingdifferent aspects of nodule
chemistry, geology, and extractivemetallurgy (e.g., Glasby, 1977;
Meylan et al., 1981, andreferences therein; Exon, 1982). More
recently a renewedinterest in marine minerals has led to the
exploration ofseamount Fe-Mn crust deposits because these are
enriched inthe currently more valuable metal Co relative to their
abyssalnodule counterparts. Work discussing seamount Fe-Mn
crustsincludes, but is not limited to, Craig et al. (1982), Halbach
etal. (1982, 1983), Halbach and Manheim (1984), Aplin andCronan
(1985a, 1985b), Hein et al. (1985a, 1985b, 1988), DeCarlo et al.
(1987a, 1987b), Pichoki and Hoffert (1987), and DeCarlo and Fraley
(1990). Summaries of nodule and crust datafrom the Australian
region have been presented by Jones(1980) and Exon et al. (in
press).
We present here the results of bulk chemical and mineral-ogical
analyses performed on a suite of Fe-Mn oxides recov-ered during
several expeditions of Rig Seismic and JOIDES
1 von Rad, U., Haq, B. U., et al., 1992. Proc. ODP, Sci.
Results, 122:College Station, TX (Ocean Drilling Program).
2 Department of Oceanography, School of Ocean and Earth Sciences
andTechnology, University of Hawaii, Honolulu, HI 96822, U.S.A.
3 Bureau of Mineral Resources, Canberra, ACT, 2601
Australia.
Resolution to the Wombat Plateau, a subplateau of the Ex-mouth
Plateau, on the Australian Northwest Shelf.
STUDY AREA AND ANALYTICAL METHODS
Regional Setting and Physical DescriptionThe ferromanganese
deposits described here all come from
the Wombat Plateau, which is a horst block on the northernmargin
of the Exmouth Plateau of northwestern Australia(Fig. 1). The crest
of the Wombat Plateau is about 1800 mbelow sea level and forms part
of the southern margin of the5600-m-deep Argo Abyssal Plain (Exon
and Willcox, 1980). Itis cut off from the Exmouth Plateau proper to
the south by ahalf-graben, where the sill water depth is about 2800
m. Theplateau is elongated east-west and the area less than 2000
mdeep is about 100 × 50 km in extent. It became a
free-standinghorst in the Late Jurassic, and a major wave-cut
unconformityseparates the Triassic and Jurassic sedimentary and
volcanicrocks from the overlying Early Cretaceous age and
youngersedimentary rocks. The Wombat Plateau has been
surveyedgeophysically by a number of vessels, and has been
exten-sively dredged and cored by the Australian Bureau of
MineralResources (BMR) using Sonne (von Stackelberg et al.,
1980)and Rig Seismic (Exon et al., 1986) as well as drilled
duringLeg 122 of the Ocean Drilling Program (ODP) using
JOIDESResolution (Haq, von Rad, et al., 1990).
Manganese crusts and nodules were dredged from thenorthern slope
of the Wombat Plateau in water depths of4600-2800 m using Rig
Seismic (Table 1). Most had formed ona substrate of Mesozoic rocks
of altered volcanic or sedimen-tary lithology (von Rad et al., this
volume). Most are cruststhat vary in thickness from mere veneers to
massive crusts 8cm thick, from poorly to well laminated, from rough
to smoothsurfaced, and from mid-brown to very dark brown in color.
Ingeneral these deposits are fairly pure Fe-Mn oxides but
clayeycalcareous layers are present in some of them. Several
gener-ations of Fe-Mn oxide deposition are well illustrated in
sampleBMR56-DR141-1.
335
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E. H. DE CARLO, N. F. EXON
Table 1. Ferromanganese nodules and crusts recovered from
theWombat Plateau.
Site (location;water depth) Description
8 ODP drill hole with Fe-Mn deposits
• Fe- Mn nodules and crusts outcropping
Figure 1. Bathymetric map showing locations of
ferromanganesedeposits, Sites 759 and 760, and BMR dredge hauls
B56-DR12 andB56-DR14 on the Wombat Plateau, Vαldiviα stations V16-3
and V16-4on the Scott Plateau, and Sonne stations S8-148, S8-165,
S8-167, andS8-170 on the Wallaby Plateau.
Manganese nodules and crusts were cored at Leg 122 Sites759 and
760 on the southern slope of the Wombat Plateau(Table 1). In
Section 122-759B-9R-1, a displaced nodule 3 cmin diameter, buried
under sediment, appears to have origi-nated from the boundary
between Upper Triassic detritalsedimentary rocks and lower Miocene
nannofossil ooze. Atthe boundary is a foraminiferal quartz sand, at
least 1.4 mthick, containing small manganese fragments and
clearlyrepresenting the onset of Cenozoic deposition after a
longperiod of erosion or nondeposition. This indicates that
thenodule formed more than 17 m.y. ago.
At Site 760, manganese nodules, as large as 5 cm indiameter, and
crusts occur in unfossiliferous fining-upwardsequences of silty
sandstone, sandy siltstone, and silty clay-stone, over a recovered
interval 6.4 m thick in Core 122-760A-9H (lithologic Unit III)
through Core 122-760A-11X(lithologic Unit IV). These Mn-bearing
strata overlie UpperTriassic marine claystone siltstone and
sandstone and underlieupper Eocene nannofossil ooze. These
relationships indicatethat they are more than 35 m.y. old. The
strata consist of a40-cm nodular Mn crust on top (Section
122-760A-9H-5), a
BMR56-DR12 All ferromanganese crusts probably formed on(16°31'S,
115O17°E Mesozoic volcanic rocks. Sample BMR56-
4600-3500 m) DR12-B2 is poorly laminated and 6.5 cm thick.Sample
BMR56-DR12-B3 is variably laminatedand 5 cm thick. The upper 3 cm
of SampleBMR56-DR12-B4 is laminated and thestructureless lower part
is 5 cm thick.
BMR56-DR14 The dredge haul contained Upper Triassic
shelf(16°33'S, 115°27'E carbonates, Cretaceous mudstones, and
3440-2690 m) Cretaceous/Paleogene chalk. SampleBMR56-DR14-I1
shows three generations ofgrowth: a 2-cm nodule/crust, a
1.5-cm-thickamorphous crust, and a 1.5-cm-thickbotryoidal crust.
Sample BMR56-DR14-I2consists of Mn crustal fragments in a matrix
ofcalcareous sandstone. SampleBMR56-DR14-G1 is a 2-mm veneer around
a2.5-cm-diameter claystone core.
ODP Site 759 Sample 122-759B-9R-1, 0-3 cm, (69 mbsf) is a
2(16°57'S, 115°34'E × 3 cm nodule with a botryoidal surface.
The
2092 m) ferromanganese portion is about 5 to 7 mmthick around a
large brown nucleus.Ferromanganese oxide (dendritic)
intergrowthsoccur in the substrate material. Thesubstrate/crust
boundary is sharp on one sidebut more diffuse on the other. The
nodule islikely of pre-Oligocene age.
ODP Site 760 The samples are from a variegated(16°55'S, 115°32'E
unfossiliferous siliciclastic sequence, 78-84
1970 m) mbsf, of probable Late Cretaceous to Eoceneage. Three
samples in Section 122-760A-9H-5are from a 40-cm-thick nodular
crust. Twosamples from Sections 122-760A-10X-1 and122-760A-11X-1
are nodules from a41-cm-thick lower layer contained insiliciclastic
sediments. The nodules are slightlybotryoidal on the surface but
generallysmooth. The Fe-Mn nodules are very shinyand black and are
surrounded by darkbrown-black ferromanganese
oxide-enrichedsediment.
360-cm variegated siliciclastic layer, a 41-cm mixed layer
ofsiliciclastic sediment and nodules (Section 122-760A-10X-1),and a
76-cm variegated siliciclastic layer (Section 122-760A-11X-1) with
an Mn nodule on top which probably fell down-hole from Section
122-760A-10X-1.
Similar nodules and crusts have been dredged from theScott
Plateau, 650 km northeast of the Wombat Plateau (Fig.1), and from
the Wallaby Plateau, 450 to 550 km to the south(Hinz et al., 1978;
von Stackelberg, 1978; von Stackelberg etal., 1980).
Possible Age of Ferromanganese Deposits
The Wombat Plateau sank to bathyal depths in the LateCretaceous,
with pelagic ooze and chalk predominating frommiddle Albian times.
Until the Oligocene, when the Circum-Antarctic Current developed,
the west wind drift drove cur-rents around northwestern Australia,
causing widespread ero-sion in the deep sea, especially in the
Paleocene to earlyEocene and the early Oligocene (Cook, 1977). Site
765 on theArgo Abyssal Plain (Ludden, Gradstein, et al., 1990)
showsfour unconformities in the Cenozoic sequence. There areseveral
unconformities produced by bathyal currents on theWombat Plateau,
and strong deep-water currents are present
336
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FERROMANGANESE DEPOSITS, WOMBAT PLATEAU
today. At Site 761, where the most complete Cretaceous-Cenozoic
section was drilled, there are unconformities be-tween the late
Neocomian to Aptian, early Eocene, lateEocene to early Oligocene,
late Miocene, and early to middlePliocene, corresponding fairly
closely to the results from Site765 on the abyssal plain.
Ferromanganese nodules and crusts can form at any un-conformity,
so on the Wombat Plateau the possibilities forformation are great.
The general opinion arising from the ODPresults is that the Wombat
Plateau was subject to sedimenta-tion during most of the Jurassic
and erosion in the LateJurassic, so manganese deposits are unlikely
to be older thanLate Jurassic. This suggests that the deposits at
Site 760formed sometime in the Cretaceous-Eocene period and thoseat
Site 759 sometime in the Cretaceous-Oligocene. Indeed,microscopic
examination of foraminifers attached to or incor-porated in Samples
122-760A-9H-5, 2-4 cm, and 122-759B-9R-1, 0-3 cm, corroborate and
refine these suggestions.Well-preserved spiny-walled forms
characteristic of theEocene (e.g., Acarinina primitiva, A.
soldadoensis gr.) wereobserved in the former sample and are
interpreted as from theearly Eocene (J. Resig, unpubl. data, 1989).
Foraminifers inthe latter sample are not as well preserved, but are
alsosuggestive of late Paleocene or Eocene organisms. It is
alsopossible that these foraminifers represent downhole
contami-nation (von Rad et al., this volume), and, hence, represent
aminimum age. Calcispheres in the sand directly underlying
themanganese crusts at Site 760 (in Sections 122-760A-9H-6
and122-760A-10X-1) have been tentatively dated as (Early)
Cre-taceous (H. Keupp, pers. comm. to U. von Rad, 1990); hence,it
is likely that the manganese crust is of Late Cretaceous toEocene
age.
The dredged ferromanganese material from the submarineoutcrops
(samples BMR56-DR12 and BMR56-DR14) alsogenerally has formed on
Mesozoic substrates. The ferroman-ganese includes very
well-preserved arenaceous foraminifers(Fig. 2) that cannot be
identified. The substrates on which thecrusts formed were probably
deeply buried at least untilcontinental breakup formed the Argo
Abyssal Plain in theearliest Cretaceous. Bathyal sedimentation
started in theAptian, so that is probably the earliest possible age
for onsetof nodule and crust formation. It is highly likely that
suchformation has continued at suitable locations until the
presentday.
Analytical Procedures
Selected representative specimens were separated fromattached
foreign or substrate material, subsampled, and ex-amined under a
binocular microscope. Air-dried portions ofthe material of interest
were manually ground to pass SPEXstandard 100-mesh sieves and
stored in glass vials for subse-quent work.
The mineralogy was studied with X-ray diffraction (XRD),using a
Scintag PAD-V diffractometer equipped with CuKαradiation and a
solid-state Ge-Li detector. Samples wereanalyzed as dried slurries
on glass slides. Samples werescanned at a rate of 2° 20/min over
the range of 2°-70°.
Chemical analyses were performed on a 110°C dry basis.Duplicate
or triplicate splits of each sample were dissolvedwith mineral
acids in a microwave oven using CEM Corpora-tion Teflon digestion
vessels. The microwave technique,adapted after Nadkarni (1984) and
Kingston and Jassie (1986),is much more efficient than either
open-vessel digestion orbomb digestion in an induction oven and
requires less reagentand only about 15 min of digestion as compared
with severalhours for the former methods. Digestion of the samples
wasperformed with HC1, HN0 3 , and HF to ensure complete
solubilization of the aluminosilicate fraction. Two
ferroman-ganese-rich sediment samples from Site 760 that contained
alarge proportion of CaCO3 were pretreated prior to
furtheranalysis. Carbonate removal was performed by a
mildlycorrosive leach in a pH 5.0 sodium acetate and acetic
acidbuffer. Rare earth elements (REE) were separated from majorand
minor constituents by ion-exchange chromatographyprior to analysis
as described by De Carlo (1990). Elementalanalysis was performed by
inductively coupled plasma (ICP)emission spectroscopy using a
Leeman Labs Plasma Spec Isequential ICP system. The relative
precision of replicatedeterminations was generally better than 5%
of the reportedvalue; accuracy was ascertained to be within 5%
relative byanalysis of U.S. Geological Survey standard Fe-Mn
nodulesA-l and P-l (Flanagan and Gottfried, 1980).
RESULTS AND DISCUSSION
MineralogyA compilation of crust, nodule, and sediment
mineralogy is
given in Table 2. The identification of mineral phases is
basedon peak positions and relative intensities as compared
tostandard reference patterns.
The manganese oxide phases found in this study are limited
tovernadite (δ-MnOi) and todorokite. Vernadite is identified bytwo
broad peaks at 2.45 and 1.42 Å (Burns and Burns, 1977;Ostwald,
1988), and represents hydrogenetic manganese oxideprecipitated
under oxidizing conditions. Todorokite, identifiedon the basis of
reflections at 9.6, 4.8, and 3.2 Å (Ostwald, 1988),was observed
only in the upper part of dredge sample BMR56-DR12-B2, but was
common in the ODP samples (six out ofeight); in all cases, however,
the amount of todorokite present isquite small relative to that of
vernadite. The macroscopic mor-phology of the todorokite-bearing
samples is similar to that of thevernadite crusts, which suggests
that the deposition of theprincipal phase, vernadite, took place
under oxidizing conditionsbut likely also was influenced by
secondary diagenetic interac-tions that led to the formation of
todorokite. It is also possiblethat the todorokite formed as a
result of postdepositional re-working of the deposits. A mixture of
vernadite and todorokite iscommon in abyssal nodules found at the
sediment/seawaterinterface; the seawater-exposed surface is more
enriched invernadite, whereas the bottom half of the nodule, buried
in thesediment, is enriched in todorokite (Bolton et al., 1990,
andreferences therein).
Goethite is the only crystalline Fe oxide phase found in
thisstudy. Its principal reflections occur at 4.19, 2.69, 2.45,
2.19,and 1.72 Å. Goethite was observed in the lower part of
sampleBMR56-DR12-B2, Sample 122-760A-10X-1, 14-16 cm, andthe inner
and outer sections of Sample 122-760A-11X-1, 7-9cm. Sample
BMR56-DR12-B2 contains significantly moregoethite than the others.
Most marine ferromanganese oxidesusually contain X-ray amorphous
FeOOH, but the occurrenceof goethite has been reported in samples
where the vernaditestructure cannot incorporate all the Fe within
the interlayersof the MnO2 (D. S. Cronan, pers. comm., 1989). Von
Stack-elberg et al. (1984) and, more recently, De Carlo (1991)
havereported the presence of goethite in certain layers of
deep-seahydrogenous crusts.
The majority of the samples also contain detrital
material(quartz, feldspars, and clay minerals) as well as
calcite.Calcite is a primary constituent of sample BMR56-DR14-I2,
aferromanganese-impregnated carbonate rock substrate with athin
outside coating of Fe and Mn oxides. Calcite is alsoabundant in the
ferromanganese oxide-bearing sediments re-covered at Site 760. The
latter also contain a significantamount of quartz.
337
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E. H. DE CARLO, N. F. EXON
Figure 2. Arenaceous foraminifers in ferromanganese crusts on
volcaniclastic rock from BMR station56-DR12. A. Chambers of several
individuals cut in random directions. B. Planispiral form cut
normalto plane of coiling. Width of photographs is 600 µm.
Chemical CompositionMajor and Minor Elements
The major and minor element composition of the samplesanalyzed
in this study is presented in Table 3. Most sampleshave a similar
overall composition except for sample BMR56-DR14-I2, which was
described previously as composed pri-marily of calcite with traces
of iron and manganese carbonatesand a patchy Fe-Mn oxide surface
coating. Excluding sampleBMR56-DR14-I2, Fe concentrations range
from 14.0% to
30.9%. A greater variability is observed in the dredged
mate-rial than in the core samples, which display a range of
16.8%to 25.8%. Manganese content varies between 13.2% and21.0%,
excluding sample BMR56-DR14-I2 (4.4% Mn).
All samples contain relatively low concentrations of themetals
of potential economic interest. Cobalt and nickelconcentrations are
lowest in sample BMR56-DR14-I2 (0.1%and 0.14%, respectively). Other
samples vary from 0.12% and0.14% to maxima of 0.55% and 0.58%,
respectively, in Sample122-760A-10X-1, 14-16 cm, which consists of
only a thin (
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FERROMANGANESE DEPOSITS, WOMBAT PLATEAU
Table 2. Mineralogy of ferromanganese nodules and crusts fromthe
Wombat Plateau, determined by powder XRD.
Sample
BMR56-
DR12-B2(upper)
DR12-B2(lower)
DR12-B3DR12-B4
(upper)DR12-B4
(lower)DR14-G1DR14-I1DR14-I2
122-759B-
9R-1, 0-3cm (nodule)
122-760A-
9H-5, 2-4 cm(nodule)
9H-5, 10-13cm(sediment)
9H-5, 10-13cm (nodule)
9H-5, 20-22cm(sediment)
10X-1, 14-16cm (nodule)
HX-l,7-9cm(outernodule)
HX-l,7-9cm(innernodule)
Major
δ-MnO2
δ-MnO2,goethite
δ-MnO2δ-MnO2
5-MnO2
δ-MnO2, quartz6-MnO2, quartzCaCO3, siderite,
rhodochrosite
δ-MnO2
δ-MnO2
δ-MnO2, quartz,CaCO3
&-MnO2, quartz,CaCO3
CaCO3, δ-MnO2,quartz
δ-MnO2, quartz
δ-MnO2,goethite,quartz
δ-MnO2,goethite,quartz
Minor or trace
Goethite, todorokite,siderite
Quartz, rhodochrositeSiderite, quartz
Quartz, goethite, siderite
CaCO3, siderite, goethiteδ-MnO2
Quartz, siderite, goethite
Quartz, CaCO3, todorokite,siderite
Siderite, todorokite
Siderite, illite, clinoptilolite
Siderite, todorokite
Todorokite, feldspars
Siderite, todorokite
Siderite, todorokite,feldspars
mm) encrustation around a siliceous substrate. The two high-est
Ni and Co concentrations are observed in the ODP nodulesamples,
whereas the unconsolidated ferromanganese-richsediments display
lower concentrations of these elements(Fig. 3). Copper ranges from
0.07% to a maximum of 0.2% ona dry-weight basis. The data in this
study indicate thatferromanganese oxides from the Wombat Plateau
probablyare of no economic interest.
Concentrations of Ca, Mg, and Ba are generally quitesimilar in
all samples and lie in a relatively narrow range.Calcium values
range from 0.63% to 2.65% excluding carbon-ate-rich Sample
BMR56-DR14-I2 (23.7%), whereas Mg andBa are 0.64%-1.3% and
0.09%-0.47%, respectively. A slightenrichment of Ba is observed in
the core samples relative tothe dredged material.
Concentrations of Al and Si in the Wombat Fe-Mn oxidesreflect
the varying amounts of detrital quartz and alumino-silicates in the
deposits. Silicon concentrations range fromvalues of 3.7% to 13.4%,
whereas Al concentrations rangefrom 1.1% to 3.5%. Samples with
greater amounts of quartz(as determined by XRD in samples
BMR56-DR14-I1 andBMR56-DR14-I2) have the highest Si concentrations
andexhibit Si: Al ratios significantly higher than the
approximately3:1 ratio commonly observed for detrital
alumino-silicates.Titanium concentrations range from 0.34% to
1.15%. Vana-dium concentrations are on the order of several hundred
partsper million and range from 190 to 980 ppm. The Mo content
ofthe Wombat Fe-Mn oxide samples is generally low, but varies
by more than an order of magnitude (50-630 ppm); this metalis
generally more abundant in the dredged samples than in thecore
samples.
The greater degree of compositional homogeneity evidentin the
core samples relative to the dredged Fe-Mn material isnot entirely
surprising, as the former were all recovered froma much more
stratigraphically and geographically constrainedarea. However, the
finding that the concentration ranges ofmost elements studied are
similar in both the dredged samplesand the core samples would
suggest that these samples formedunder similar depositional (e.g.,
generally highly oxic) condi-tions. This suggests that, in a very
broad sense, depositionalconditions conducive to the formation of
ferromanganesedeposits existing over the Wombat Plateau may not
havechanged very much over the last 30-40 m.y. The most
notabledifferences in elemental abundance between samples
collectedby dredging and the ferromanganese layers drilled during
Leg122 lie in the generally greater Ba, Co, Fe, and Ni abundancesin
the latter. An opposite trend is observed for Mo, which ismore
enriched in the dredged samples. The compositionaldifferences in
these elements could perhaps be related to agradual deepening of
the area since the Neocomian and/or tochanges in the water
temperature due to post-Eocene globalcooling, although the latter
would be counteracted in part byAustralia^ movement into
subtropical regions since breakupwith Antarctica. Predominating
currents at the time whendeposition of the cored Fe-Mn oxides began
were affected bythe west wind drift, which drove currents around
northwest-ern Australia. These waters were likely enriched in
nutrientsand possibly trace elements, and thus local upwelling
mayhave caused higher productivity by providing a source ofelements
to the accumulating Fe-Mn deposits. It is also likelythat as
bottom-water circulation changed as a result of theevolving
geologic environment of the Wombat Plateau, waterswith slightly
different compositions influenced the formationof the dredged
deposits (which continue to accrete to this day)and may have
contributed to their slightly different composi-tions.
Interelement Correlations
Interelement correlations for the 16 samples from this studywere
calculated in a matrix containing 14 variables. The data setwas
also divided into two subsets representing the dredgedsamples and
those recovered by coring in order to evaluaterelationships within
each group. Because of the small number ofsamples analyzed, use of
the correlation matrices to evaluateinterelement relationships
should be made with caution.
The following positive correlations, in decreasing order,were
found for the entire data set: (r > 0.8) Fe with V, and Cowith
Ni and Zn; (r > 0.7) Zn with Cu, and Ba with Ni and Al;and (r
> 0.6) Mg with Mn, Co with Al and Ba, and Zn with Aland V. All
other positive correlations display r values lessthan 0.6 (Co, Mo,
and Ni with Mn; Cu and Zn with Fe; Bawith Ti; and Mg with Ni).
Strong negative correlations (r > -0.7) are observed
betweenCa and the three metals Fe, Mn, and V. These correlations
likelyresult from the diluting effect of CaCO3 on the Fe-Mn
fraction inthe deposits (Halbach and Puteanus, 1984). Slightly
weakernegative correlations between Mo and Al (r = -0.698)
andbetween Zn and Ca (r = -0.627) suggest that both Mo and Zn
areassociated with the Fe-Mn oxide phase of the deposits,
althoughneither shows strong correlations with Fe or Mn. Most
interele-ment correlations found in this sample set are in
accordance withassociations previously observed in marine Fe-Mn
deposits. Asomewhat surprising association is that of Ba with Al
becausethe former is also correlated with Co and Zn.
339
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E. H. DE CARLO, N. F. EXON
Table 3. Major and minor element composition of selected
ferromanganese nodules and crusts.
Sample
BMR56-
DR12-B2 (upper)DR12-B2 (lower)DR12-B3 (bulk)DR12-B4
(upper)DR12-B4 (lower)DR14-G1DR14-I1 (bulk)DR14-I2
aAverage
Standard deviation
122-759B-
9R-l ,0-3cm(nodule)
122-760A-
9H-5, 2-4 cmb9H-5, 10-13 cm9H-5, 10-13 cm(nodule)
b9H-5, 20-22 cm10X-1, 14-16 cm
(nodule)c HX- l ,7 -9cm
(outer)c HX- l ,7 -9cm
(inner)
Average
Standard deviation
Fe
19.7230.8720.3321.2119.1114.2413.964.87
19.92
5.63
17.70
18.8625.8422.29
20.8816.84
22.99
22.02
20.93
3.00
Mn
20.6513.2516.2021.0415.3814.4414.744.41
16.52
3.08
16.42
15.7613.1513.55
14.2216.30
13.94
17.17
15.06
1.52
Mn/Fe
1.0470.4290.7970.9920.8051.0141.0560.905
0.881
0.195
0.928
0.8360.5090.608
0.6810.968
0.606
0.780
0.740
0.165
Co
0.3190.2320.1660.2000.2620.1240.3180.057
0.232
0.074
0.353
0.4650.2760.325
0.3300.548
0.325
0.496
0.390
0.099
Ni
0.3620.1600.2210.3390.2170.2500.3150.139
0.266
0.074
0.351
0.5780.2970.346
0.3090.575
0.304
0.459
0.402
0.119
Cu
0.0700.2000.1120.0720.0700.0760.1680.028
0.110
0.054
0.111
0.1850.1310.131
0.1190.159
0.116
0.085
0.130
0.031
Zn
0.0610.0790.0500.0610.0570.0510.0630.024
0.060
0.010
0.080
0.1090.0980.086
0.0800.099
0.087
0.084
0.090
0.010
Ca
2.081.481.662.211.492.281.81
23.7
1.85
0.33
1.75
1.320.632.65
2.261.65
0.88
1.03
1.52
0.69
Mg
1.070.9530.9701.090.9150.9600.9690.636
0.987
0.067
0.899
1.280.7860.836
0.8361.09
0.714
0.793
0.904
0.188
Ba
0.1030.2380.1190.1090.2690.0860.1190.027
0.149
0.073
0.219
0.3290.2920.341
0.4670.272
0.308
0.275
0.313
0.073
Si
5.323.668.514.598.089.78
13.46.02
7.62
3.40
8.89
7.738.947.52
9.288.52
9.66
6.35
8.36
1.09
Al
1.361.651.551.142.502.091.362.01
1.66
0.48
3.12
3.542.742.62
3.223.25
2.48
2.66
2.95
0.38
Ti
0.7610.9100.4610.6121.1530.4830.4560.342
0.690
0.266
0.815
0.5430.6960.689
0.7610.793
0.758
1.021
0.760
0.135
V
0.0710.0980.0690.0830.0700.0590.0690.019
0.074
0.013
0.086
0.0770.0990.089
0.0750.053
0.092
0.075
0.081
0.014
Mo
0.0430.0370.0530.0630.0420.0270.0450.005
0.044
0.011
0.037
0.0130.0100.007
0.0080.012
0.005
0.013
0.013
0.010
Note: All sample concentrations expressed in weight percent on
110°C dried basis.a Calculated excluding Sample BMR56-DR14-I2.
Sediment from interval treated with pH = 5 buffer for carbonate
removal.c Nodule was separated into its outer and inner
portions.
0.6
0.4-
0.2 0.4Co (%)
0.6
Figure 3. Scatter diagram of Co (%) vs. Ni (%) in Wombat
PlateauFe-Mn deposits. Squares = BMR dredged samples; circles =
ODPcore samples.
Breakup of the data into subsets (see Fig. 4) revealsintragroup
relationships that are not immediately apparent inthe composite
data set. It also allows us to compare the twosample groups and
better identify differences between them.This approach reveals that
although the two sample groups aresimilar in their chemistry, they
are generally distinguishablethrough their respective elemental
compositions (Figs. 4A-4F). For example, there appears to be no
relation between Aland Fe for the dredged samples, whereas the core
samplesdisplay a strong inverse correlation between these
elements
(Fig. 4A). A notable exception to this trend is the good
linearrelation between V and Fe for the entire sample set (Fig.
4E),although the dredged samples contain less V than the
coresamples. Positive correlations between Ni and Mg (r = 0.783and
0.852) in each subgroup (Fig. 4F) are stronger than in thecombined
data (r = 0.558) and stronger than that reported byHein et al.
(1988) for Fe-Mn crusts from the Marshall Islands.However, the
correlations contrast with observations in theHawaiian Archipelago
(De Carlo et al., 1987a), where theseelements display no
interdependence. These findings suggest avariability in the
processes that influence the supply of Ni tothe various areas.
There may also be a small detrital input ofNi (from enrichment in
volcanic Mg-silicates) in the Wombatsamples as observed by Hein et
al. (1988) in crusts from theMarshall Islands. On the other hand,
Ni in Hawaiian crusts isprimarily of hydrogenetic origin (De Carlo
et al., 1987a).
Bolton et al. (1990) found a correlation between Ti and Fein
abyssal nodules (r = 0.99) that is much stronger thanobserved here
(/• < 0.63). This may result from an enrichmentof TiO2 in the
interlayer FeOOH of vernadite in smoothabyssal nodules (Halbach and
Ozkara, 1979). The Ti insamples from this study is more strongly
associated with Fethan Mn in the dredged samples, whereas the
opposite is truein the core samples.
The associations of Co, Ni, V, and Mo with Mn aregenerally
stronger in the dredged subset matrix, reflect theinfluence of
depth-dependent hydrogenetic enrichment pro-cesses (Halbach and
Puteanus, 1984), and are consistent withother studies (Aplin and
Cronan, 1985a; Halbach et al., 1982;Hein et al., 1985b, 1988; De
Carlo et al., 1987a, 1987b). Figure3 also shows that Co and Ni are
strongly associated with eachother, with slightly more scatter
observed in the dredged data
340
-
FERROMANGANESE DEPOSITS, WOMBAT PLATEAU
3.5
3.0
Co 2.5
< 2.0
1.5
1.0
0.6
0.5
Q 0-4
Z 0.3
0.2
O.I
° 0
-
E. H. DE CARLO, N. F. EXON
Table 4. Rare earth element composition of ferromanganese
nodules and crusts.
Core, section,interval (cm)
122-759B-b9R-l, 0-3
122-760A-
9H-5, 2-4c9H-5, 10-13b9H-5, 10-13c9H-5, 20-2210X-1, 14-1611X-1,
7-9 (outer)11X-1, 7-9 (inner)
dNASC
La
69
122166154184152159226
32
Ce
2058
1350191517601925167019902460
73
Pr
14.1
22.229.627.031.625.427.536.87.9
Nd
50.4
90.812211813411011414833
Sm
4.49
13.818.816.121.014.816.718.45.7
Eu
0.80
1.333.022.242.292.722.563.961.24
Gd
17.2
21.327.824.631.425.527.835.4
5.2
Tb
16.2
11.416.415.616.016.014.418.60.85
Dy
8.51
14.417.816.420.817.117.022.0
5.8
Ho
1.45
2.863.182.903.903.533.014.061.04
Er
5.74
9.2510.410.814.012.110.714.03.4
Tm
1.121.33
1.50
1.321.440.5
Yb
3.79
7.518.338.02
10.19.578.02
11.03.1
Lu
0.70
1.461.451.881.801.691.481.950.48
Ceanomalya
7.17
2.792.942.922.702.873.232.88
Note: Concentrations expressed in µg/g solid phase on a 110°C
dried basis.a Defined as 2(Ce/Ce*)/(La/La* + Pr/Pr*), where *
indicates the shale value.
Untreated nodule.c Sediment leached with sodium acetate/acetic
acid buffer.
North American Shale Composite from Haskin et al. (1968).
rents. The proposed Paleocene-Eocene age of this nodulesuggests
that cold oxidizing water moving northward alongwestern Australia
bathed the Wombat Plateau and enhancedthe development of highly
oxic conditions.
Another feature observed in the REE patterns is a signifi-cant
positive Gd anomaly that is larger than those previouslyidentified
by Hein et al. (1988) in the Marshall Islands and byDe Carlo (1990)
in a thick Fe-Mn crust from Schumannseamount near the Hawaiian
Archipelago. Probably the mostunusual feature of the REE patterns
in this study is theapparent depletion of the elements Nd, Sm, and
Eu in Sample122-759B-9R-1, 0-3 cm, and to a lesser extent Sm and Eu
inthe other samples relative to their near-neighbor elements Prand
Gd (Fig. 5). These results are not likely caused by
spectralinterferences in the ICP technique, and the extent of
depletionvaries from sample to sample. However, the low
concentra-tions of Eu in the analyzed solutions does lead to a
largerrelative error for this element than for many other REE.
Aslight upward trend for the heavy REE Yb and Lu that can
beattributed to the incorporation of apatite (e.g., De Carlo,
1991)in the ferromanganese is unlikely here because no apatite
wasidentified by XRD.
Comparison with Other Marine Fe-Mn DepositsA comparison of
average elemental abundances in the
samples from this study with those of other marine Fe-Mndeposits
is presented in Table 5. The data in this table indicatethat the
Wombat Plateau samples (excluding BMR56-DR14-I2due to its highly
anomalous composition) are generally similarto other Fe-Mn deposits
from plateau settings in the Austra-lian region. The Mn content is
quite consistent throughout theAustralian region except for the
samples recovered from theCape Leeuwin nodule field. This is not
surprising as these aredeep-sea nodules (4300-5300 m) that resemble
deep-sea Pa-cific Ocean deposits more and typically have a much
greaterabundance of todorokite and hence a higher Mn/Fe ratio
thanthe plateau deposits, which tend to be more enriched
invernadite. A greater variability is observed for Fe, and
itsconcentration is more elevated in samples from this study
thanfrom other sites such as the Tasman Sea rises, WallabyPlateau,
and Coral Sea. However, both the Scott Plateau andthe South Tasman
Rise deposits display nearly identical Fecontents to those found
here. Metals of potential commercialinterest (Co, Cu, and Ni)
exhibit generally quite low concen-trations throughout the
Australian region (generally below1%) in comparison with the higher
average values (near 1.4%)
found for deep-sea nodules and especially with those of
thehigh-grade (2.5%-3%) Clarion-Clipperton nodule belt of thenorth
equatorial Pacific Ocean (Cronan, 1980).
The detrital mineral content of the deposits from this study,as
measured by their Al, Ca, Mg, Si, and Ti contents, is inagreement
with other studies except that South Tasman Risedeposits display
more elevated Ca and Al concentrations. Nosignificant differences
in the other metals analyzed existbetween the samples from this
study and the available data inTable 5.
CONCLUSIONSFerromanganese crusts and nodules from the Wombat
Plateau are primarily vernadite-rich deposits with minoramounts
of todorokite and goethite. The deposits are charac-terized by a
low Mn/Fe ratio (average = 0.77, range =0.43-1.05) and exhibit
relatively low concentrations of thetransition metals of potential
commercial interest (Co + Cu +Ni = 0.68%). They are quite similar
to other plateau Fe-Mncrusts and nodules from the Australian region
and are of nocommercial interest.
The results of microscopic examination of the
calcareousorganisms in the sediments, crusts, and nodules cored
duringLeg 122 suggest that they formed in the Late Cretaceous
toEocene. Samples recovered during BMR cruise 56 may havebegun to
accumulate as early as the Late Cretaceous andferromanganese oxide
accumulation is likely to have contin-ued through the present.
Rare earth elements in the ODP core samples generallyexhibit
lower concentrations than those in Pacific seamountFe-Mn crusts but
are characterized by high Ce anomalies. Thehighest Ce anomaly
(7.17) and lowest REE abundances wereobserved in Sample
122-759B-9R-1, 0-3 cm. These findingssuggest that this sample may
have formed under extremelyoxidizing conditions enhanced by the
presence of strongbottom currents.
ACKNOWLEDGMENTSWe are grateful to the captains and crews of Rig
Seismic
and JOIDES Resolution for their assistance at sea. We
alsoexpress our appreciation to the co-chief scientists of
thecruises as well as to our shipboard colleagues.
Technicalassistance was provided by D. Koeppenkastrop, W.
Shibata,and K. Mitchell. Ulrich von Rad provided photographs
andlithologic descriptions of the Rig Seismic dredge samples.
Weacknowledge reviews by V. Marchig and D. Puteanus-Stube
342
-
FERROMANGANESE DEPOSITS, WOMBAT PLATEAU
1.2-
0.4
A
1.6
Lα Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
RARE EARTH ELEMENTS
Figure 5. Shale-normalized REE patterns of Fe-Mn-rich samples
fromthe Wombat Plateau. A. Squares = Sample 122-759B-9R-1, 0-3
cm;triangles = Sample 122-760A-9H-5, 2-4 cm; and diamonds =
Sample122-760A-10X-1, 14-16 cm. B. Open triangles = Sample
122-760A-9H-5, 10-13 cm (SASED); solid triangles - Sample
122-760A-9H-5,10-13 cm (NOD); and squares = Sample 122-760A-9H-5,
20-22 cm(SASED). C. Inner (solid symbols) and outer (open symbols)
portionsof Sample 122-760A-11X-1, 7-9 cm. SASED is sodium
acetate/aceticacid buffer leached sediment, NOD is an untreated
nodule.
which greatly improved this manuscript. Research support toE. De
Carlo was provided through a U.S. Science AdvisoryCommittee (USSAC)
ODP post-cruise grant. N. Exon pub-lishes with the permission of
the Director, Bureau of MineralResources, Canberra. Any opinions,
findings and conclusionsor recommendations expressed in this
publication are those ofthe authors and do not necessarily reflect
the views of theNational Science Foundation, Joint Oceanographic
Institu-tions, Inc., or Texas A&M University. This is SOEST
con-tribution no. 2394.
0.4
0.4
0.8
0.4
A HAWAIIAN ARCHIPELAGO
i i i i i i i i i i i i i i
B KIRIBATI AND TUVALU
i i i i i i i i i i i i i
Lα Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
RARE EARTH ELEMENTS
Figure 6. Comparison of shale-normalized average REE
abundancepatterns for marine Fe-Mn deposits. A. Average and range
of crustsfrom the Hawaiian Archipelago (De Carlo et al., 1987a). B.
Averageand range of crusts from Kiribati (De Carlo and Fraley,
1990). C.Average of crusts from this study of the Wombat
Plateau.
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Date of initial receipt: 18 April 1990Date of acceptance: 19
February 1991Ms 122B-183
344
-
FERROMANGANESE DEPOSITS, WOMBAT PLATEAU
Table 5. Summary of the elemental composition (weight %) of
ferromanganese nodules and crusts from the Australian region.
Location(water
depth, m)
MnFeNiCuCoNi + Cu + CoZnPbSiAlTiMgCaNaKPBaVMo
This studya
(2690-4600)
15.7720.450.340.120.220.680.08
_8.012.350.730.941.68
-__
0.240.080.028
South TasmanRiseb
(1600-4000)
15.0119.220.390.160.330.880.080.149.515.660.711.413.740.490.731.05__-
Tasman Searisesc
(1550-2530)
14.6612.920.390.310.270.970.140.14
10.722.650.531.501.740.340.430.41__-
Cape Leeuwindeep-sea
nodule fieldd
(4300-5300)
20.311.90.940.280.121.340.080.06_
2.130.461.661.23_
0.29_
0.030.040.03
Pacific Oceanaveragee
19.7811.960.630.390.331.350.070.088.323.060.671.711.962.050.750.230.280.050.04
Indian Oceanaveragee
15.1014.740.460.290.230.980.070.09
11.402.490.66_
2.37___
0.180.040.03
ScottPlateauf
(1933-2587)
18.218.50.380.050.330.76
WallabyPlateau8
(2500-4300)
12.9212.830.290.110.170.57
Coral Seah
(978-2555)
13.813.10.260.060.290.62
a Fifteen nodule and crust samples.b Eighteen nodule and crust
samples (Bolton et al., 1988).c Four nodule and crust samples
(Bolton et al., 1990).d Seven nodule samples (Pettis and de Forest,
1979).e Cronan (1980).
Eight nodule and crust samples (Hinz et al., 1978).8
Thirty-three nodule and crust samples (von Stackelberg et al.,
1980).h Six nodule and crust samples (Exon et al., in press).
345
